We present a structural model for amyloid fibrils formed by the 40-residue ␤-amyloid peptide associated with Alzheimer's disease (A␤ 1-40), based on a set of experimental constraints from solid state NMR spectroscopy. The model additionally incorporates the cross-␤ structural motif established by x-ray fiber diffraction and satisfies constraints on A␤ 1-40 fibril dimensions and mass-per-length determined from electron microscopy. Approximately the first 10 residues of A␤ 1-40 are structurally disordered in the fibrils. Residues 12-24 and 30 -40 adopt ␤-strand conformations and form parallel ␤-sheets through intermolecular hydrogen bonding. Residues 25-29 contain a bend of the peptide backbone that brings the two ␤-sheets in contact through sidechain-sidechain interactions. A single cross-␤ unit is then a double-layered ␤-sheet structure with a hydrophobic core and one hydrophobic face. The only charged sidechains in the core are those of D23 and K28, which form salt bridges. Fibrils with minimum mass-per-length and diameter consist of two cross-␤ units with their hydrophobic faces juxtaposed.A myloid fibrils are filamentous structures, with typical diameters of Ϸ10 nm and lengths up to several micrometers, formed by numerous peptides and proteins with disparate sequences and molecular weights. Biomedical interest in amyloid fibrils arises from their occurrence in amyloid diseases (1), including Alzheimer's disease, type 2 diabetes, Huntington's disease, and prion diseases. Current interest in the molecular structures of amyloid fibrils additionally arises from fundamental questions regarding the molecular mechanism of amyloid formation and the nature of the intermolecular interactions that stabilize these structures for an extremely diverse class of polypeptides.No high-resolution molecular structure of an amyloid fibril has yet been determined experimentally because amyloid fibrils are noncrystalline solid materials and are therefore incompatible with x-ray crystallography and liquid state NMR. X-ray fiber diffraction shows that amyloid fibrils contain cross-␤ structural motifs, i.e., extended ␤-sheets in which the ␤-strand segments run approximately perpendicular to, and the intermolecular hydrogen bonds run approximately parallel to, the long axis of the fibril (2, 3). Other molecular-level structural features of amyloid fibrils are not well established.In the case of fibrils formed by the full-length ␤-amyloid peptide associated with Alzheimer's disease (A␤), which ranges from 39 to 43 residues in length in vivo (4, 5), several molecular models have been proposed (6-10). These models exhibit many qualitative and quantitative differences, reflecting the paucity of experimental constraints. All of these models are inconsistent with recent measurements of 13 C-13 C nuclear magnetic dipole-dipole couplings (i.e., intermolecular distances) by solid state NMR (11-13), which imply an in-register parallel alignment of peptide chains within the cross-␤ motif in A␤ 1-40 and A␤ 1-42 fibrils (A␤ mϪn denotes residues m t...

Graphite, as the most common anode for commercial Li-ion batteries, has been reported to have a very low capacity when used as a Na-ion battery anode. It is well known that electrochemical insertion of Na þ into graphite is significantly hindered by the insufficient interlayer spacing. Here we report expanded graphite as a Na-ion battery anode. Prepared through a process of oxidation and partial reduction on graphite, expanded graphite has an enlarged interlayer lattice distance of 4.3 Å yet retains an analogous long-range-ordered layered structure to graphite. In situ transmission electron microscopy has demonstrated that the Na-ion can be reversibly inserted into and extracted from expanded graphite. Galvanostatic studies show that expanded graphite can deliver a high reversible capacity of 284 mAh g À 1 at a current density of 20 mA g À 1 , maintain a capacity of 184 mAh g À 1 at 100 mA g À 1 , and retain 73.92% of its capacity after 2,000 cycles.

The seven-residue peptide N-acetyl-Lys-Leu-Val-Phe-Phe-Ala-Glu-NH(2), called A beta(16-22) and representing residues 16-22 of the full-length beta-amyloid peptide associated with Alzheimer's disease, is shown by electron microscopy to form highly ordered fibrils upon incubation of aqueous solutions. X-ray powder diffraction and optical birefringence measurements confirm that these are amyloid fibrils. The peptide conformation and supramolecular organization in A beta(16-22) fibrils are investigated by solid state (13)C NMR measurements. Two-dimensional magic-angle spinning (2D MAS) exchange and constant-time double-quantum-filtered dipolar recoupling (CTDQFD) measurements indicate a beta-strand conformation of the peptide backbone at the central phenylalanine. One-dimensional and two-dimensional spectra of selectively and uniformly labeled samples exhibit (13)C NMR line widths of <2 ppm, demonstrating that the peptide, including amino acid side chains, has a well-ordered conformation in the fibrils. Two-dimensional (13)C-(13)C chemical shift correlation spectroscopy permits a nearly complete assignment of backbone and side chain (13)C NMR signals and indicates that the beta-strand conformation extends across the entire hydrophobic segment from Leu17 through Ala21. (13)C multiple-quantum (MQ) NMR and (13)C/(15)N rotational echo double-resonance (REDOR) measurements indicate an antiparallel organization of beta-sheets in the A beta(16-22) fibrils. These results suggest that the degree of structural order at the molecular level in amyloid fibrils can approach that in peptide or protein crystals, suggest how the supramolecular organization of beta-sheets in amyloid fibrils can be dependent on the peptide sequence, and illustrate the utility of solid state NMR measurements as probes of the molecular structure of amyloid fibrils. A beta(16-22) is among the shortest fibril-forming fragments of full-length beta-amyloid reported to date, and hence serves as a useful model system for physical studies of amyloid fibril formation.

Increasing evidence suggests that formation and propagation of misfolded aggregates of 42-residue human amyloid β (Aβ(1–42)), rather than the more abundant Aβ(1–40), provokes the Alzheimer’s cascade. To date, structural details of misfolded Aβ(1–42) have remained elusive. Here we present the atomic model of Aβ(1–42) amyloid fibril based on solid-state NMR (SSNMR) data. It displays triple parallel-β-sheet segments that are different from reported structures of Aβ(1–40) fibrils. Remarkably, Aβ(1–40) is not compatible with the triple-β motif, as seeding with Aβ(1–42) fibrils does not promote conversion of monomeric Aβ(1–40) into fibrils via cross-replication. SSNMR experiments suggest that the Ala42 carboxyl terminus, absent in Aβ(1–40), forms a salt-bridge with Lys28 as a self-recognition molecular switch that excludes Aβ(1–40). The results provide insight into Aβ(1–42)-selective self-replicating amyloid propagation machinery in early-stage Alzheimer’s disease.

The detailed chemical structure of graphite oxide (GO), a layered material prepared from graphite almost 150 years ago and a precursor to chemically modified graphenes, has not been previously resolved because of the pseudo-random chemical functionalization of each layer, as well as variations in exact composition. Carbon-13 (13C) solid-state nuclear magnetic resonance (SSNMR) spectra of GO for natural abundance 13C have poor signal-to-noise ratios. Approximately 100% 13C-labeled graphite was made and converted to 13C-labeled GO, and 13C SSNMR was used to reveal details of the chemical bonding network, including the chemical groups and their connections. Carbon-13-labeled graphite can be used to prepare chemically modified graphenes for 13C SSNMR analysis with enhanced sensitivity and for fundamental studies of 13C-labeled graphite and graphene.

Diffusible subfibrillar aggregates of amyloid proteins are potent neurotoxins and primary suspects in amyloid diseases including Alzheimer's disease. Despite widespread interest, the molecular structures of the amyloid intermediates and the conformational conversions in amyloid misfolding are poorly understood. Here we present a molecular-level examination of sequence-specific secondary structures and supramolecular structures of a neurotoxic amyloid intermediate of the 40-residue β-amyloid (Aβ) peptide involved in Alzheimer's disease. Using solid-state NMR and electron microscopy, we show that, before fibrillization, natively unstructured monomeric Aβ is subject to large conformational changes into a spherical amyloid intermediate of 15–35 nm diameter, which has predominantly parallel β-sheet structures. Structural comparison with Aβ fibrils demonstrates that formation of this β-sheet intermediate I(β) largely defines conformational transitions in amyloid misfolding. Neurotoxicity assays on PC12 cells show that I(β) shows higher toxicity than the fibril, indicating that the β-sheet formation may trigger neurotoxicity.

No abstract

A technique is presented to recouple homonuclear dipolar couplings between dilute spin pairs such as 13 C-13 C systems under very fast magic angle spinning ͑MAS͒ in solid-state nuclear magnetic resonance ͑NMR͒ spectroscopy. The presented technique, finite pulse rf driven recoupling ͑fpRFDR͒, restores homonuclear dipolar interactions based on constructive usage of finite pulse-width effects in a phase-and symmetry-cycled-pulse train in which a rotor-synchronous pulse is applied every rotation period. The restored effective dipolar interaction has the form of a zero-quantum dipolar Hamiltonian for static solids, whose symmetry in spin space is different from that obtained by conventional rf driven recoupling ͑RFDR͒ techniques. It is demonstrated that the efficiency of recoupling by fpRFDR is not strongly dependent on chemical shift differences or resonance offsets in contrast to previous recoupling methods under very fast MAS. To realize distance measurements without effects of spin relaxation, a constant-time version of fpRFDR ͑CT-fpRFDR͒ is introduced, in which the effective evolution period is varied by refocusing dipolar evolution with a rotor-synchronized solid echo while the total recoupling period is kept constant. From CT-fpRFDR experiments at a spinning speed of 30.3 kHz in a field of 17.6 T, the 13 C-13 C distance of ͓1-13 C͔Ala-͓1-13 C͔Gly-Gly was determined to be 3.27 Å, which agrees well with the value of 3.20 Å obtained by x-ray diffraction. Also, two-dimensional ͑2D͒ 13 C/ 13 C chemical-shift correlation NMR spectrum in a field of 9.4 T was obtained with fpRFDR for fibrils of the segmentally 13 C-and 15 N-labeled Alzheimer's ␤-Amyloid fragments, A␤ 16-22 ͑residues 16-22 taken from the 40-residue A␤ peptide͒ in which Leu-17 through Ala-21 are uniformly 13 C-and 15 N-labeled. Most 13 C resonances for the main chain as well as for the side chains are assigned based on 2D 13 C/ 13 C chemical-shift correlation patterns specific to amino-acid types. Examination of the obtained 13 C chemical shifts revealed the formation of ␤-strand across the entire molecule of A␤ 16-22. Possibility of high throughput determination of global main-chain structures based on 13 C shifts obtained from 2D 13 C/ 13 C chemical-shift correlation under very fast MAS is also discussed for uniformly/segmentally 13 C-labeled protein/peptide samples.